Chapter 12: The Sun and the Earth

Modern people cherish the Sun. On the few days when the Sun appears in Seattle, everyone—and I mean everyone—goes outside and occupies a patch of grass, a park bench, or any other sunny spot they can find. (You know it’s true, Seattleites.) And surely, you can make a case that Southern Californians worship the Sun, what with their devotion to sunglasses, suntans, sunroofs, and sunny dispositions. College football teams (e.g., Arizona Sun Devils), brands of food (e.g., Sun Chips), and the most sacred day of the week—Sunday—are all named after the Sun. Even the cars we drive—the Solstice, Equinox, Eclipse, and Solara—refer to the Sun.

Our solar devotion has deep historical roots. Consider:

• The Polynesian word Maui refers to a god who captured the Sun for their people at Haleakala Crater on Maui, Hawaii, one of the best places in the world to view a sunrise (Westervelt 1910; Andersen 1995).
• The Incans and Aztecs worshiped the Sun, as evidenced in the temples at Qorikancha and Machu Picchu in Peru and the Pyramid of the Sun in the ancient city of Teotihuácan near Mexico City (e.g., Cowgill 2015).
• The Ise Grand Shrine complex in Ise, Japan, including Naikū and 91 nearby Shinto shrines devoted to Amaterasu-ōmikami, the goddess of the Sun, has existed since 4 BCE (e.g., Akima 1993). 
• The Konark Sun Temple in Konark, India, built in the 13th century in honor of the sun god Surya, attracts hundreds of thousands of visitors each year (Behera 2005). 
• A 68-foot-tall temple dedicated to Ra in Cairo marks the site of the ancient Egyptian city Heliopsis, the City of the Sun (Dobrowolska and Dobrowolski 2006).
• Stonehenge, now believed to be an ancient burial site, was built in alignment with the positions of the sunrise and sunset on the first day of summer in England (e.g., Pearson et al. 2009).
• At Mesa Verde, the Anasazi built a Sun Temple some 800 years ago, possibly as an astronomical observatory (e.g., Towers 2016). 

But we must go deeper than this. Did you know that the Sun drives the motions of the ocean and atmosphere, creates our weather, and drives the production of food, lumber, medicines, and oxygen? Did you realize that every time you drive an internal combustion car, you unleash ancient sunlight, energy from the Sun that fell on Earth millions of years ago (e.g., Hartmann 2004)? Fossil fuels—sunlight captured by ancient plants and algae that were buried and transformed into coal, natural gas, and crude oil—drive the world’s economies. The Sun powers our planet, feeds our hunger, and governs our daily lives. In many ways, our lives revolve around the Sun just as in ancient times. 

But our modern relationship with the Sun has a dark side. Human activities have disrupted the natural processes by which the Sun makes Earth habitable for life by maintaining a narrow range of climate-comfortable temperatures. In unleashing ancient sunlight, we’ve unleashed carbon dioxide to our atmosphere—lots of it. As a result, our planet is warming. Human-caused warming has brought extreme weather, sea level rise, and the unraveling of ecosystems. How we deal with these threats in the next couple decades may well determine how and where we live. 

If we are to live on this planet for centuries to come, then perhaps we owe it to ourselves and future generations to have a basic grasp of the Sun and how it provides energy to our planet and civilization. Find a sunny place and dive in.

12.1 Interesting Facts About the Sun

Our Sun, a star in the Milky Way galaxy, formed about 4.6 billion years ago. It began as a protostar, a whirl of gases—namely helium and hydrogen—that collapsed into a spinning ball. Increased temperature and pressure within the ball over the course of some 50 million years eventually ignited hydrogen fusion, a reaction that sustains the release of energy from the Sun even today. Under extreme gravitational forces, hydrogen atoms fuse into helium atoms, resulting in the release of enormous quantities of energy. The start of nuclear fusion represents the transition from protostar to full-fledged star (e.g., NASA 2023b). It is the moment when we may say, “Let there be sunlight, the sum of all wavelengths of light emitted by the Sun.” From that moment on, light from the Sun—sunlight—filled space.

12.1.1 The Solar Wind

In addition to light, the Sun also emits charged particles—electrons, protons, and others—in a steady stream known as the solar wind. These particles interact with Earth’s magnetic field and gases in Earth’s atmosphere to create spectacular light shows, the aurora borealis and aurora australis, the northern and southern lights, respectively. Coronal mass ejections (CMEs)—intense outbursts of particles from the Sun—occasionally shift the lights toward the equator, offering opportunities for people living at lower latitudes to witness auroras (e.g., NASA 2019b). 

CMEs can also produce dangerously strong geomagnetic storms, disruptions of Earth’s magnetic field. In September 1859 the solar wind was so powerful it produced an aurora visible around the world. Electrical pulses from the event flickered through telegraph systems, shocking telegraph operators and setting their papers on fire (e.g., Muller 2014). While the damage that day was limited, scientists estimate that if such a storm were to occur today, the cost could reach trillions of dollars (Riley et al. 2018). NASA and other agencies monitor and forecast solar activity, what’s known as space weather. Like atmospheric weather forecasts, space weather forecasts allow officials to inform civilian and military populations of possible disruptions to satellites, electrical power, computers, and other electrical devices during strong solar activity.

12.1.2 Classification of the Sun

Different stars in the Universe have different characteristics. When you look at stars in the sky, they may appear white, yellow, red, or blue. It’s not an optical illusion. Different stars emit different colors of light. Astronomers can use these colors (and other properties) to classify stars—kind of like sorting your socks or dress shirts by color. Star classification serves as a kind of shorthand for describing the characteristics of a star. Based on the properties of the light and particles emitted, astronomers classify our Sun as a G2V star, a yellow dwarf star, and a main sequence star (NASA 2021). These classifications have great significance for astronomers, but for our purposes, the important point is that the Sun emits white-yellow light.

Of course, the Sun is not the only star in our neighborhood. Peer into the night sky and you will see thousands of twinkling stars. On a really dark night, far from the lights of industrial civilization, you might see as many as 4,500 stars. With a pair of binoculars or a telescope, you can see as many as a half a million stars and galaxies—collections of star systems bound together by gravity. Our own galaxy—the Milky Way—contains some 200 billion stars. And beyond the Milky Way, in the vast expanse of the Universe, lie perhaps as many as two trillion other galaxies, each with its own billions and billions of stars. It’s said there are more stars in the Universe than grains of sand on the world’s beaches. Of course, an accurate count of the number of stars in the Universe or grains of sand on Earth’s beaches can never be known for sure. But it’s fun to think about.

12.1.3 The Solar System

Around the time that our Sun lit up, gases and cosmic materials in nearby space coalesced into the celestial bodies orbiting the Sun, our solar system. The rocky planets—Mercury, Venus, Earth, and Mars—and the gas planets—Jupiter, Saturn, Uranus, and Neptune, formed around this time. So did the dwarf planet, Pluto, and hundreds of moons and asteroids. At the far reaches of the solar system, a collection of icy objects, the Kuiper Belt and the Oort Cloud, appeared as well. These icy bodies occasionally send traveling snowballs our way, the comets (NASA  2019a).

The dimensions of our Sun are impressive. It makes up 99.98 percent of all the matter in our solar system. It has a diameter that is 109 times the diameter of Earth. If the Sun were a hollow sphere, you could put 1.3 million Earth spheres inside of it. Of course, if you put Earth inside the Sun, you would experience heat like you’ve never felt before. The core of the Sun has a temperature of about 27 million degrees Fahrenheit (F), about 15 million degrees Celsius (C); °F or °C, respectively. Out at its edge, it’s cooler, only 10,000°F (5,500°C; e.g., NASA 2021).

People sometimes ask why the Sun looks so small if it’s that big. Well, if you were 93 million miles away, you’d look small, too. This distance—the distance from the Earth to the Sun—represents an astronomer-designated unit called the astronomical unit, or AU. Earth, by definition, is one AU from the Sun. But because the Earth orbits the Sun in a slightly elliptical path, the Earth–Sun distance varies. In January, the Sun is slightly closer to Earth, at perihelion, about 0.98 AU (91.4 million miles). In July, it’s slightly farther away, at aphelion, about 1.02 AU (94.5 million miles). On average, it works out to about 93 million miles (NASA 2001). 

This brief introduction to the star of our solar system serves as a backdrop for the more important consideration of how the Sun—specifically, energy from the Sun—interacts with Earth and drives so many important Earth processes. Let’s take a look now.

12.2 Electromagnetic Radiation

Energy from the Sun comes to us in the form of electromagnetic radiation, a term that refers to all the types of radiant energy emanating from stars (e.g., the Sun). Scientists classify different types of electromagnetic radiation according to their wavelengths, the distance between the peaks of their waveform (just like sound waves). The arrangement of different types of electromagnetic radiation according to their wavelengths produces what is known as the electromagnetic spectrum, the range of all types of electromagnetic radiation. From longest to shortest, the electromagnetic spectrum consists of radio waves (the waves you listen to), microwaves (the waves you cook with), infrared light (used in night-vision gear), visible light (so named because you can see it), ultraviolet light (the reason you use sunscreen), X-rays (used by doctors and dentists), and gamma rays (used in medicine; e.g., NASA 2016b). In everyday usage regular folks just refer to the Sun’s electromagnetic spectrum as sunlight, solar radiation, or solar energy. But the spectral divisions are useful, too, as we shall see. 

12.2.1 Energy

Most people recognize that light is a form of energy. Few, however, appreciate what the word energy means. Formally, energy is defined as “the ability to do work”—crudely speaking, to move something (e.g., EIA 2023c). I like to think of energy as the stuff that makes things happen. It’s a property of an object or system that can be used to generate a force of some kind and do work.

Broadly speaking, two kinds of energy exist: (1) kinetic energy, the energy of motion; and (2) potential energy, stored energy or energy inherent in an object in a gravitational field. Kinetic energy includes solar radiation (electromagnetic energy), the motion of objects (motion energy), sound waves (sound energy), electricity (electrical energy), and heat exchange (thermal energy). Potential energy may refer to energy stored in chemical bonds (chemical energy), energy stored in objects under tension (mechanical energy), radioactive materials (nuclear energy), or the force experienced by Humpty Dumpty on his epic fall (gravitational energy; EIA 2023a). 

More important than the definition of energy is how it operates. Like Mystique of the X-People, energy can shape-shift. It can change from one form to another. Scientists express energy’s ability to change forms in the well-known law of conservation of energy. In nontechnical terms, the law of conservation of energy states that energy can be neither created nor destroyed, but it can change forms. That means that one type of energy can become another type of energy—energy can transform. It’s still the same energy; it’s just in a different package (e.g., EIA 2023b). 

Familiar examples will help you grasp this rather abstract concept. A log, the dried-out hunk of a trunk of a tree, was formed when the tree captured electromagnetic energy—sunlight. The tree turned the sun’s energy into chemical bonds, the energy-containing stuff of which the tree is made. When you set the log ablaze, you transform its chemical energy into heat energy that provides warmth. A burning log also releases electromagnetic energy, the red-yellow-orange glow of its fire. And when you heat a marshmallow over a fire and eat it (with chocolate and graham crackers, of course),  your body transforms their chemical energy into the kinetic energy of a laugh, a fist bump, or a romantic kiss. The story of energy is the story of its transformations. 

12.2.2 Interactions of Light with Matter

Interactions between light (i.e., electromagnetic radiation) and matter (i.e., the atoms of which materials are composed) prove important for understanding transformations of energy. Fortunately, we need only master a few simple definitions to appreciate these interactions. 

To understand light–matter interactions, it helps to think of light as rays, arrows that indicate the direction of a light wave. When a light ray encounters an object, many possible things may happen. But let’s consider the four interactions that play a role in our understanding of how solar radiation heats our planet:

• absorption, the energy in the light ray is transferred to the object and becomes part of the object’s energy
• scattering, the light ray interacts with the object and is sent in different directions
• reflection, the light ray bounces off the object at the same angle at which it arrived
• transmission, the light ray travels through an object and some or all of its energy comes out the opposite side from which it entered

Now, these definitions may not be picture perfect in the eyes of  physicists, but they hopefully give you an understanding of the basic principles. (See also Ahrens and Henson for a good basic treatment of this topic.)

12.2.3 Temperature and Heat

When an object absorbs electromagnetic radiation, it gains energy. This causes its molecules to move more rapidly. In scientific terms, the kinetic energy of the molecules increases. We detect a gain in energy as a rise in temperature, loosely defined as the average kinetic energy of a system, substance, or object. Temperature provides a convenient way to compare the kinetic energy of systems, substances, and objects. 

To be clear, temperature is not a measure of heat energy, total kinetic energy, or even the internal energy in a system or object. It’s simply a convenient way to describe the jumpiness or sluggishness of the molecules in a system or object. By analogy, if kids in a bounce house are jumping all over the place, you might say their temperature is high. If the kids are barely moving, their temperature is low. The higher the temperature, the faster the molecules move, and vice versa. It’s important to note that measurements of temperature are not the same as measurements of energy. An ice cube and an iceberg may have the same temperature, but the iceberg, by virtue of its much larger mass, contains considerably more energy. 

12.2.4 Thermometers

A thermometer—a device used to measure temperature—provides a number for the average kinetic energy in an object or system. The level of fluid in a thermometer rises and falls according to the temperature. In a digital thermometer, a digital display corresponds to the expansion or contraction of a strip of metal or changes in its electrical conductivity as temperature changes. 

The numbers displayed by a thermometer are completely arbitrary. In the Celsius scale—invented in 1742 by Swedish astronomer Anders Celsius (1701–1744)—the temperature of freezing water is set arbitrarily at 0°. In the Fahrenheit scale—invented in 1724 by physicist David Gabriel Fahrenheit (1686–1736)—the temperature of freezing water is set arbitrarily at 32°. The numbers for the freezing points of water were chosen by Mr. Celsius and Mr. Fahrenheit when they invented their thermometers. The same is true for the boiling points, 100°C and 212°F. These numbers were chosen by their inventors who for various reasons thought that they worked well for the range of temperatures that people who use the thermometers would encounter (e.g., Middleton 1966).

Thermometers work by heat exchange between the external environment and a fluid inside a tube. (Traditionally, the fluid was mercury; now most liquid thermometers contain alcohol.) When the external environment is hotter than the fluid inside the thermometer, heat energy flows from the environment to the fluid, and the fluid expands. The fluid expands because the molecules in the fluid are moving faster, and they take up more space in their motions. We see the fluid rise in the tube. When the external temperature is lower than the fluid inside the thermometer, heat energy flows from the fluid to the external environment; the fluid loses energy and the space between the molecules is reduced—the fluid contracts. We see the level of the fluid fall in the tube. Similar expansions and contractions—or changes in electrical conductivity—accompany heat exchange between the external environment and digital thermometers.

Regardless of the chosen scale, thermometers provide temperature measurements according to agreed-upon methods. They provide us with a means for determining the relative hotness or coolness of objects or systems. They ensure consistency and reproducibility in processes that depend on temperature control, such as cooking, the manufacture of steel, and a whole lot more.

12.2.5 Heat Transfer 

Heat only exists when two objects or systems have different temperatures. Think of heat as energy in motion. When two objects or systems reach thermal equilibrium—when they have the same temperature—heat no longer exists (e.g., Ahrens and Henson 2018).

The increase in temperature that occurs when matter absorbs electromagnetic radiation represents one of three ways that heat can be transferred between objects or systems: (1) radiation, (2) conduction, and (3) convection. All three forms of heat transfer become important when we examine what happens when Earth’s surface absorbs sunlight. 

Radiation is the transfer of heat via electromagnetic energy. Radiation moves out in all directions from its source until it meets another object. That object may reflect the radiation, in which case no exchange of energy occurs; it may transmit the radiation, in which case the radiation travels right through it and no exchange of energy occurs; or it may absorb the energy, in which case the energy of the object (and its temperature) increases. If you’ve ever felt the hood of a black car on a sunny day, you’ve likely noticed that it’s hotter than the surrounding air. That’s because the metal of a car’s hood absorbs electromagnetic radiation, increasing its temperature. When you touch the hot hood, heat energy is transferred to your finger, and you utter some variation of “ow” (depending on how much it hurts). 

A car’s hood also emits infrared radiation, electromagnetic energy at wavelengths beyond visible light. You do not detect this kind of radiation because, unlike visible light (which you can see), your eyes are not sensitive to infrared radiation. All objects, including humans, emit radiation in proportion to their temperature. Hotter objects emit shorter wavelengths, and cooler objects emit longer wavelengths. The relationship between temperature and the wavelength of maximum emission defines a principle known as Wien’s Law. This law provides a quantitative formula for determining the peak emission wavelength from the temperature of an object. As temperature rises, the peak emission wavelength becomes shorter. 

The infrared emission of objects, humans, and other forms of life makes them detectable using night-vision technologies. These devices have sensors that detect infrared radiation and electronics that turn the infrared signal into a color the user can see—usually green. They are very useful for seeing in the dark, but generally they are used for clandestine or military purposes. 

Conduction occurs when the faster molecules of a higher-temperature object collide with the slower molecules of a lower-temperature object and exchange energy until all the molecules are moving at the same speed (i.e., until they have the same kinetic energy). Conduction is the transfer of heat energy through molecule-to-molecule collisions. Heat transfer by conduction requires contact between two objects. When you put a marshmallow on a metal skewer in a campfire, the end immersed in the fire gains energy and begins to transfer that energy via conduction along the length of the skewer. If you keep it in the fire long enough, the end you’re holding will eventually get too hot and burn you. Skewers made of wood—a poor conductor of heat—protect your hand from burning. Kitchen utensils employ principles of heat conduction to prepare food and to protect chefs. Pans made of copper, aluminum, and carbon steel conduct heat faster than those made of cast iron or stainless steel. Fast-conducting pans are great for searing foods. At the other end of the pan, pot handles made of plastic or other non-conducting materials—or a good pair of cotton pot holders—limit heat conduction and prevent your hands from burning.

Convection involves heat transfer through the motions of fluids, such as air and water. When you heat air or water, you cause it to expand. This expansion lowers the density of the fluid. If the surrounding fluid is more dense, then the heated fluid will rise. This should be familiar to you as the principle behind heating and cooling in your home—heating devices (or their ducts) are generally on or near the floor because warm air rises. Air conditioners (or their ducts) are generally placed at the top of a window or in the ceiling because cold air sinks. The rising or sinking air transfers heat through its movements, which is called convective heating. Convection causes air or water to circulate—to move in a circular motion. On a global scale, convection is responsible for the large-scale motions of the atmosphere and the ocean—the global winds and ocean currents. (See Ahrens and Henson 2018.)

12.3 How the Sun Warms the Earth

As we learned earlier, sunlight is a form of electromagnetic radiation. We most commonly observe it as visible light with wavelengths from 400 to 700 nanometers (10^–9 meters, or 1 billionth of a meter). These wavelengths include the colors we see: violet, blue, green, yellow, orange, and red. To understand how the Sun warms the Earth, we also need to consider nonvisible forms of light—wavelengths shorter than 400 nanometers, namely ultraviolet radiation, and wavelengths longer than 700 nanometers, namely infrared light.

Because the gases in Earth’s atmosphere absorb different wavelengths of electromagnetic radiation, some wavelengths of sunlight travel freely through Earth’s atmosphere and others don’t. The selectivity of the atmosphere to different wavelengths of sunlight is referred to as the atmospheric window (NWS 2023).Think of it as a kind of selective filter, blocking some wavelengths of light but not others. If you look closely at an illustration of Earth’s atmospheric window, you will see that not all sunlight reaches Earth’s surface and not all wavelengths of sunlight reach Earth’s surface equally. Gamma rays and X-rays are mostly blocked by Earth’s atmosphere. Ultraviolet, visible, and near-infrared wavelengths are transmitted through Earth’s atmosphere. Mid- to far-infrared wavelengths are blocked. Short- to mid-wavelength radio waves are transmitted, while long-wavelength radio waves are blocked. The blocking (or transmitting) is due to different gases in Earth’s atmosphere. The net effect is that about 20 percent of the Sun’s energy is absorbed in the atmosphere and only about 50 percent reaches Earth’s surface (e.g., Trenberth et al. 2009).

Other than radio waves, the atmosphere is most transparent (least opaque) to visible light (as to be expected given that our eyes evolved to detect this spectrum of light). In contrast, the part of the spectrum known as near infrared (700 to 5,000 nanometers) varies in its transmission through the atmosphere. And mid- to far-infrared wavelengths (5,000 to 350,000 nanometers) are nearly 100 percent blocked by the atmosphere.

Visible and mid- to far-infrared light are the operational parts of the electromagnetic spectrum where heating of our planet is concerned. To simplify the discussion, scientists divide these parts of the spectrum into two parts: shortwave radiation, which includes ultraviolet, visible, and near-infrared wavelengths; and longwave radiation, which includes the mid- to far-infrared wavelengths. Shortwave radiation is transmitted by Earth’s atmosphere. Most longwave radiation is absorbed by Earth’s atmosphere (e.g., NASA 2016a).

The Sun, like all stars, emits all forms of electromagnetic radiation. However, most of the radiation emitted by the Sun occurs in wavelengths between 300 and 2,500 nanometers. In fact, about half the radiation that reaches Earth’s surface from the Sun consists of ultraviolet and visible wavelengths. The other half is near-infrared wavelengths. Thus, the sunlight that reaches Earth’s surface can be classified as shortwave radiation. However, the Earth, like all objects that contain energy, also emits radiation. According to Wien’s Law, the wavelengths of the emitted radiation depend on the temperature of the object: Hot objects emit short-wavelength radiation, while cool objects emit long-wavelength radiation. The superhot Sun emits mostly shortwave radiation, while the much cooler Earth emits longwave radiation. It’s Earth’s emission of longwave radiation and reabsorption of that radiation by gases in Earth’s atmosphere that cause the atmosphere to warm.

12.3.1 The Greenhouse Effect

The filtering of solar radiation discussed above results from gases in our atmosphere. While nitrogen (70 percent) and oxygen (21 percent) make up the bulk of the gases, they absorb very little solar radiation. However, small concentrations of other atmospheric gases absorb significant quantities, especially longwave radiation. Four gases in particular are responsible for absorbing most of the longwave radiation emitted by Earth’s surface: carbon dioxide (CO₂), water vapor (H₂O), methane (CH₄), and nitrous oxide (N₂O). The similarity of these gases to the function of a greenhouse (trapping heat) has earned them the nickname greenhouse gases (NASA 2023c). Scientists refer to the warming of Earth’s surface by the greenhouse gases in the atmosphere as the greenhouse effect. (Watch 17-second animation by NASA.)

Operationally, Earth’s atmosphere acts similarly to a greenhouse except that it doesn’t prevent convection and confine heat like a greenhouse. Shortwave radiation—free to travel through Earth’s atmosphere—reaches Earth’s surface, where it is absorbed. Absorption of that radiation raises the temperature of Earth’s surface. In turn, some of that energy is emitted by Earth’s surface to the atmosphere as longwave radiation. In the atmosphere, that surface-emitted longwave radiation is absorbed by—you guessed it—greenhouse gases. As a result, they heat up. As they heat up, the greenhouse gases re-radiate some of their absorbed radiation. In doing so, they warm the atmosphere and Earth’s surface. In a sense, greenhouse gases recycle longwave radiation, acting as a reservoir of heat that keeps Earth’s surface warm.

Lest you get the impression that the greenhouse effect is bad, know that the natural greenhouse effect is a good thing—even an essential thing. Because of the greenhouse effect, Earth experiences an average surface temperature of roughly 59°F (15°C). Without the greenhouse effect, Earth’s average surface temperature would be 0°F (−18°C). 

12.3.2 Atmospheric Scattering

While the greenhouse effect is the principal means by which Earth warms, other processes can diminish or enhance the amount of sunlight that reaches Earth’s surface. When sunlight encounters the outermost part of Earth’s atmosphere, it begins to interact with suspended particles and gases. Clouds (which are suspended water and ice), particles, and atmospheric gases may absorb, reflect, and scatter sunlight. Preferential scattering of blue wavelengths by atmospheric gases makes the sky appear blue. Similarly, scattering produces those gorgeous orange-red sunsets we often observe (e.g., NASA 2022). 

But scattering can also limit the amount of sunlight that reaches Earth’s surface. The most effective light-scattering agents in the atmosphere are aerosols, a group of solid or liquid particles less than a tenth the width of a human hair (less than 1 micrometer). Aerosols prove important in the formation of clouds—acting as cloud condensation nuclei—sites of condensation for water vapor. Aerosols—a form of air pollution—originate from a number of manmade and natural sources, including transportation (e.g., trains, planes, and automobiles), industrial activities (e.g., coal burning), agricultural practices (e.g., fertilizers), volcanoes (e.g., gas emissions), mineral dust (suspended by winds), ocean waves (e.g., sea foam), and even phytoplankton, which produce chemicals that act as cloud condensation nuclei (e.g., Boucher et al. 2013). 

One type of aerosol—black carbon, formed from the incomplete combustion of fossil fuels—has received a lot of attention for its role in reducing the reflectivity of Arctic ice, causing it to melt faster (e.g., Hansen and Nazarenko 2004; Winiger et al. 2019). In the lower atmosphere, black carbon and aerosol pollution have severe health effects (e.g., Liu et al. 2018; Kusumaningtyas et al. 2018; Groma et al. 2022). On the other hand, aerosols in the upper atmosphere may actually be cooling Earth. Scientists estimate that cooling by as much as 0.9 to 1.98°F (0.5 to 1.1°C) has occurred because of atmospheric aerosols (e.g., Samset et al. 2018). Ironically, as efforts to reduce air pollution take hold, we can expect additional (and rapid) increases in Earth’s surface temperature. Decreases in aerosol emissions during the pandemic were likely responsible for increases in surface temperatures in eastern China (e.g., Yang et al. 2020). Nevertheless, uncertainty remains over the role of aerosols in Earth’s climate. Ongoing research and new approaches promise to improve our understanding in this important area of research (e.g., Li et al. 2022).

12.3.3 Albedo: The Reflectivity of Earth’s Surface 

Shortwave radiation traveling through Earth’s atmosphere may also be reflected. Reflection changes the direction of a beam of light—similar to scattering (although it does not involve light–particle interactions). Reflection of sunlight by clouds, aerosols, particles, and Earth’s surface reduces the energy heating Earth’s surface. Reflection and backscattering (scattering in the opposite direction from which light rays arrive) reduce the sunlight reaching Earth’s surface by about 30 percent. Add 20 percent absorption by clouds, particles, and atmospheric gases, and you can see why only about half the sunlight at the top of Earth’s atmosphere makes its way to Earth’s surface (e.g., Trenberth et al. 2009; Liang et al. 2019). 

Of course, the amount of sunlight reflected from Earth’s surface depends on the properties of its surface. Scientists define the reflectivity of Earth’s surface by a property known as albedo—the ratio of reflected light to incoming light. Light-colored surfaces reflect a greater quantity of light than dark-colored surfaces. Thus, light-colored surfaces have a higher albedo than dark surfaces. Places with snow and ice have a high albedo and reflect more sunlight than the ocean or land, which have a low albedo. Urban areas have variable albedos, depending on the materials used to build them and the presence (or absence) of vegetation. For example, Downtown Los Angeles has a higher albedo than surrounding areas because it has less vegetation, which tends to absorb sunlight (e.g., Taha 1997; Vahmani and Ban-Weiss 2016).

12.3.4 Negative and Positive Feedback Loops

Processes internal to a system that modify how it responds to change are known as feedback loops. Feedback loops can slow down or speed up changes in a system. The Earth system and its seven subsystems (Chapter 1) exhibit numerous feedback loops. Of prime concern here are the ones that affect Earth’s energy budget.

Feedback loops that reduce or reverse the impact of a change are known as negative feedback loops.  The thermostat in your house provides a familiar example. When the house gets hot, the A/C kicks in. When the desired temperature is reached, the thermostat turns the A/C off. Negative feedback loops reduce extremes in a system. They help to maintain more constant and stable conditions. Scattering by atmospheric particles represents a negative feedback loop because combusion of fossil fuels produces particles that reflect shortwave radiation and reduce the warming effects of the resultant greenhouse gases. If we decrease our burning of fossil fuels, we’ll produce fewer particles and Earth will warm as more shortwave radiation reaches Earth’s surface. Of course, reduced concentrations of greenhouse gases (and the resultant cooling) will more than compensate for any increases in heating.

Changes that occur in a system that amplify or accelerate an effect are known as positive feedback loops (positive as in the same direction, not the “You’re doing a great job” kind of positive feedback). During ice ages, when a greater proportion of Earth is covered with ice, more sunlight is reflected from Earth’s surface. The increased albedo leads to even more cooling and more ice and even more cooling. Alternatively, losses of ice due to warming of Earth’s surface lower Earth’s albedo and allow more sunlight to be absorbed. Greater warming means even greater loss of ice and even more warming. This type of positive feedback worries climate scientists, as global warming will cause melting of the ice caps and greater warming. 

At some point, feedback loops within a system (especially positive feedback loops) may cause it to spin out of control, so to speak. The system becomes unstable in its current state and reorganizes into a different form. Scientists refer to such events as tipping points, a change in the state of a system that is irreversible and unstoppable. A roller coaster makes a good example. As the car ascends to the highest point on the track—click–clack, click–clack, click–clack—you nervously anticipate what’s ahead. You reach the top, your wits still about you, but then, there’s that moment when the car crosses the point of no return. You’ve reached the tipping point. OMG! The entire coaster lets go and you scream at the top of your lungs, praying you’re not about to die.

Climate scientists fear the Earth system has multiple irreversible tipping points (e.g., Heinze et al. 2021; Franzke et al. 2022). The predicted impacts will be severe: permanent loss of ice sheets, disruption of ocean circulation, and mass extinction of corals, among them. In the scientists’ words, “the evidence from tipping points alone suggests that we are in a state of planetary emergency: both the risk and urgency of the situation are acute” (Lenton et al. 2019).

12.3.5 Earth’s Temperature Is Rising

While atmospheric greenhouse gases occur naturally as a result of volcanic activity, decomposition of organic matter, evaporation of water, and oceanic exchanges (among others), these natural processes cannot account for the measurably significant rise in greenhouse gases since 1850. Human activities, especially the burning of fossil fuels (i.e., petroleum, natural gas, and coal) and deforestation, the removal of trees, have added greenhouse gases to the atmosphere. 

For the 800,000 years prior to the industrial era (about 1850), atmospheric CO₂ never exceeded 300 parts per million (e.g., Lindsey 2022). Fast-forward 170 years, and the concentration of atmospheric greenhouse gases has risen to more than 400 parts per million, an increase of nearly 50 percent. On April 26, 2022, atmospheric CO₂ reached a record-setting 422.06 parts per million at the Mauna Loa Observatory in Hawaii (CO2.earth 2021). Atmospheric CO₂ has now reached its highest concentration in 23 million years (Cui et al. 2020).

What happens to Earth’s surface temperature when the concentration of greenhouse gases in the atmosphere rises? As you might predict, the addition of greenhouse gases to the atmosphere increases the amount of radiation absorbed by greenhouse gases and re-emitted to Earth’s surface. If more longwave radiation remains in our atmosphere, then it stands to reason that Earth’s temperature will rise. That’s exactly what we see. Climate scientists calculate that Earth has warmed by an average of nearly 2.0°F (1.1°C) over temperatures from 1850–1900 (IPCC 2021). That’s the warmest Earth has been in 125,000 years (Tollefson 2021).

One way to visualize what happens as a result of an increase in atmospheric CO₂ is to consider what it’s like on an ordinary day at the mall versus the mall during holidays. On an ordinary day, you can walk through the mall without bumping into people. On busy shopping days, you have to push and shove your way through crowds of people. Heat rays moving through a crowded greenhouse gas atmosphere take longer to travel into space and Earth’s heat reservoir fills up.

12.4 Earth’s Energy Imbalance

Another way to understand the greenhouse effect is to look at the atmosphere, land, and ocean as reservoirs of heat. With increasing amounts of greenhouse gases, the sources of heat—the surface-emitted longwave radiation and its absorption and re-emission by greenhouse gases—exceed the sinks of heat—the escape of longwave radiation into outer space. As a result, the amount of heat in the reservoir increases—like water in a bathtub. In the language of the reservoir model introduced in Chapter 11, the sources (E_in) exceed the sinks (E_out), or

E_in > E_out

(Eq. 12.1)

This imbalance between the incoming radiation from the Sun (i.e., the source) and outgoing radiation from Earth’s surface to space (i.e., the sink) is called Earth’s energy imbalance (e.g., Hansen et al. 2005). Estimates of the energy imbalance indicate that nearly 1 watt of energy per square meter (1 joule per second per square meter of Earth’s surface) remain in the Earth system versus what is leaving via outgoing radiation to space (e.g., Meyssignac et al. 2019; von Schuckman et al. 2020). In short, Earth is accumulating energy. According to the August 2021 report of the Intergovernmental Panel on Climate Change (better known as the IPCC):

• Every decade for the last four decades has been warmer than the one before. 2011–2020 was warmer than 2001–2010, which was warmer than 1991–2000, which was warmer than 1981–1990 (IPCC 2021).
• The last two decades (2001–2020) experienced the most rapid warming, about 1.8°F (1°C) higher than the period 1850–1900 (IPCC 2021).
• The highest temperature rise occurred over land, about 2.86°F (1.59°C); ocean temperatures rose 1.58°F (0.88°C; IPCC 2021).
• The Arctic is warming two to four times faster than the rest of the globe (e.g., AMAP 2021; IPCC 2021; Labe et al. 2020).

The rate of change of human-caused global warming is unlike anything observed in the past 2,000 years (IPCC 2021). Earth’s average temperature has been rising at a rate of about 0.13°F (0.07°C) per decade since 1880. However, since 1981, that rate has more than doubled to 0.32°F (0.18°C) per decade (Lindsey and Dahlman 2023). It’s getting warmer faster.

12.4.1 Global Warming vs. Global Climate Change

In Chapter 1, we noted that scientists have observed a change in Earth’s climate (i.e., average weather conditions) over the past 150 years or so, what is known as global climate change. Global warming is one of the changes scientists have observed. Unfortunately, in the popular media, the terms global climate change and global warming are often used interchangeably. Let’s make it clear: their meanings are distinct.

The increase in Earth’s average temperature—global warming—results primarily from increases in human-generated atmospheric greenhouse gases (though other activities also contribute). The increase in Earth’s energy alters the natural functioning of Earth’s subsystems and causes climate to change. The cryosphere melts. The atmosphere gains heat and moisture. The ocean warms. Global warming is a part of Earth’s changing climate, but many other weather- and climate-related phenomena—melting ice caps, extreme precipitation, droughts, more frequent and stronger hurricanes—also serve as examples of global climate change. The distinction between global warming and global climate change proves important for communicating the multiple changes that are happening as Earth warms. Global warming refers to a (specific) human-caused change. Global climate change refers to the myriad of weather and climate changes that occur as a result of global warming.

One other important note of clarification concerns the oft-remarked-upon (and mistaken) association of local changes with global ones. You may have heard someone comment during a cold-weather spell or snowstorm that global warming can’t be happening because it’s cold or snowing outside. Well, colder weather still happens in a warmer world. Global warming has not eliminated the seasonal cycle. Winter will still be colder than summer.

Temperature records reveal the trend expected in a warming world. The number of days of record heat exceeds the number of days of record cold. In the United States, record high temperatures occur twice as often as low temperature records (Meehl et al. 2009; Associated Press 2019). In Australia, that ratio is 12 to 1 (Lewis and King 2015). When you add it all up—across the entire globe—Earth is getting warmer. And we’re the cause of it.

12.4.2 The Ocean’s Temperature Is Rising

Because warming air temperatures affect us directly, you’d be forgiven if you hadn’t thought about the effects of global warming on the ocean. But in fact, the effects of global warming on the ocean are perhaps more alarming than those on land. That’s because the ocean has absorbed most of the heat that otherwise would have ended up in the atmosphere. A warming world ocean signals profound changes for the ocean and will literally change the world as we know it.

Science writers LuAnn Dahlman and Rebecca Lindsey refer to the ocean as Earth’s “largest solar energy collector” (Lindsey and Dahlman 2020). As early as 1956, famed meteorologist Carl-Gustaf Rossby (1898–1957) postulated that “anomalies in heat probably can be stored and temporarily isolated in the sea” (Rossby 1959). In 2000, Sydney Levitus and colleagues published the first estimates of the amount of heat being absorbed by the upper ocean (to depths less than 9,842 feet, or 3,000 meters; Levitus et al. 2000). Their results confirmed Rossby’s hypothesis: Most heat from global warming has gone into the ocean. Subsequent studies using observations from Argo floats and other newly available data substantiated the results more definitively: The world ocean has absorbed 93 percent of the excess heat associated with Earth’s energy imbalance since 1971 (Levitus et al. 2012; Rhein et al. 2013; Meyssignac et al. 2019; Cheng et al. 2023).

Oceanographers refer to ocean-absorbed energy as ocean heat content, the amount of heat energy stored within the ocean (Cheng et al. 2017). Cheng et al. (2020) estimate that since 1960, ocean heat content across all depths of the world ocean has increased by 370 ± 81 zettajoules (1 zettajoule = 10²¹ joules). Now, I admit that it’s difficult to wrap your head around joules much less zettajoules. But Levitus and colleagues (2012) calculated that if the excess heat in the upper 6,562 feet (2,000 meters) was instantaneously released—240 zettajoules since 1955 using their data—the atmosphere would warm by 65°F (36°C). For all depths of the world ocean—using the estimates of Cheng et al. 2020—that temperature comes close to 100°F (37.7°C). Of course, an instantaneous release of heat from the ocean to the atmosphere is impossible. Nevertheless, this calculation underscores the tremendous amount of heat added to the world ocean as a result of human-caused global warming. 

Upper ocean heat content set new records in 2021 and 2022, the warmest in human history (Cheng et al. 2023). Not surprisingly, ocean surface temperatures in different locations also set records. In August 2020, the sea surface temperature at Scripps Pier in La Jolla, California, reached 79.5°F (26.4°C)—11.5°F above average (6.4°C)—beating a record set only two years previously (Anderson 2020). The long-term trend in ocean warming caused by climate change accounts for these temperatures (e.g., Fumo et al. 2020).

To conclude, the ocean stores tremendous quantities of heat. The good news is the presence of an ocean on our planet means that Earth’s temperature has risen more slowly than it might have otherwise. The bad news is that even if we were to somehow stop greenhouse gas emissions today, it would take centuries for the ocean and atmosphere to return to normal. That’s because the ocean circulates on timescales of centuries to millennia (Abram 2018; Cheng et al. 2020). So heat stored in the ocean will remain with our planet for a long time. Such a realization makes it ever more urgent that we act immediately to reduce greenhouse gas emissions and put Earth on a path toward a more sustainable and habitable future.

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